package com.ctsi.commons.util.id;


import java.util.Random;

/**
 * @version 1.0
 * @author David Beaumont, Copyright 2005
 * <p>
 * A Java implementation of the MT19937 (Mersenne Twister) pseudo
 * random number generator algorithm based upon the original C code
 * by Makoto Matsumoto and Takuji Nishimura (see
 * <a href="http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html">
 * http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html</a> for
 * more information.
 * <p>
 * As a subclass of java.util.Random this class provides a single
 * canonical method next() for generating bits in the pseudo random
 * number sequence.  Anyone using this class should invoke the public
 * inherited methods (nextInt(), nextFloat etc.) to obtain values as
 * normal.  This class should provide a drop-in replacement for the
 * standard implementation of java.util.Random with the additional
 * advantage of having a far longer period and the ability to use a
 * far larger seed value.
 * <p>
 * This is <b>not</b> a cryptographically strong source of randomness
 * and should <b>not</b> be used for cryptographic systems or in any
 * other situation where true random numbers are required.
 * <p>
 * <!-- Creative Commons License -->
 * <a href="http://creativecommons.org/licenses/LGPL/2.1/"><img alt="CC-GNU LGPL" border="0" src="http://creativecommons.org/images/public/cc-LGPL-a.png" /></a><br />
 * This software is licensed under the <a href="http://creativecommons.org/licenses/LGPL/2.1/">CC-GNU LGPL</a>.
 * <!-- /Creative Commons License -->
 * 
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 *     
 * <Work rdf:about="">
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 * <License rdf:about="http://creativecommons.org/licenses/LGPL/2.1/">
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 * 
 */
public class MTRandom extends Random {

	/**
	 * Auto-generated serial version UID.  Note that MTRandom does NOT
	 * support serialisation of its internal state and it may even be
	 * necessary to implement read/write methods to re-seed it properly.
	 * This is only here to make Eclipse shut up about it being missing.
	 */
	private static final long serialVersionUID = -515082678588212038L;

	// Constants used in the original C implementation
	private final static int UPPER_MASK = 0x80000000;
	private final static int LOWER_MASK = 0x7fffffff;

	private final static int N = 624;
	private final static int M = 397;
	private final static int MAGIC[] = { 0x0, 0x9908b0df };
	private final static int MAGIC_FACTOR1 = 1812433253;
	private final static int MAGIC_FACTOR2 = 1664525;
	private final static int MAGIC_FACTOR3 = 1566083941;
	private final static int MAGIC_MASK1   = 0x9d2c5680;
	private final static int MAGIC_MASK2   = 0xefc60000;
	private final static int MAGIC_SEED    = 19650218;
	private final static long DEFAULT_SEED = 5489L;

	// Internal state
	private transient int[] mt;
	private transient int mti;
	private transient boolean compat = false;

 

	/**
	 * The default constructor for an instance of MTRandom.  This invokes
	 * the no-argument constructor for java.util.Random which will result
	 * in the class being initialised with a seed value obtained by calling
	 * System.nanoTime().
	 */	
	public MTRandom() { }

	/**
	 * This version of the constructor can be used to implement identical
	 * behaviour to the original C code version of this algorithm including
	 * exactly replicating the case where the seed value had not been set
	 * prior to calling genrand_int32.
	 * <p>
	 * If the compatibility flag is set to true, then the algorithm will be
	 * seeded with the same default value as was used in the original C
	 * code.  Furthermore the setSeed() method, which must take a 64 bit
	 * long value, will be limited to using only the lower 32 bits of the
	 * seed to facilitate seamless migration of existing C code into Java
	 * where identical behaviour is required.
	 * <p>
	 * Whilst useful for ensuring backwards compatibility, it is advised
	 * that this feature not be used unless specifically required, due to
	 * the reduction in strength of the seed value.
	 * 
	 * @param compatible Compatibility flag for replicating original
	 * behaviour.
	 */
	public MTRandom(boolean compatible) {
		super(0L);
		compat = compatible;
		setSeed(compat?DEFAULT_SEED:System.nanoTime());
	}

	/**
	 * This version of the constructor simply initialises the class with
	 * the given 64 bit seed value.  For a better random number sequence
	 * this seed value should contain as much entropy as possible.
	 * 
	 * @param seed The seed value with which to initialise this class.
	 */
	public MTRandom(long seed) {
		super(seed);
	}

	/**
	 * This version of the constructor initialises the class with the
	 * given byte array.  All the data will be used to initialise this
	 * instance.
	 * 
	 * @param buf The non-empty byte array of seed information.
	 * @throws NullPointerException if the buffer is null.
	 * @throws IllegalArgumentException if the buffer has zero length.
	 */
	public MTRandom(byte[] buf) {
		super(0L);
		setSeed(buf);
	}

	/**
	 * This version of the constructor initialises the class with the
	 * given integer array.  All the data will be used to initialise
	 * this instance.
	 * 
	 * @param buf The non-empty integer array of seed information.
	 * @throws NullPointerException if the buffer is null.
	 * @throws IllegalArgumentException if the buffer has zero length.
	 */
	public MTRandom(int[] buf) {
		super(0L);
		setSeed(buf);
	}

	// Initializes mt[N] with a simple integer seed. This method is
	// required as part of the Mersenne Twister algorithm but need
	// not be made public.
	private final void setSeed(int seed) {

		// Annoying runtime check for initialisation of internal data
		// caused by java.util.Random invoking setSeed() during init.
		// This is unavoidable because no fields in our instance will
		// have been initialised at this point, not even if the code
		// were placed at the declaration of the member variable.
		if (mt == null) mt = new int[N];

		// ---- Begin Mersenne Twister Algorithm ----
		mt[0] = seed;
		for (mti = 1; mti < N; mti++) {
			mt[mti] = (MAGIC_FACTOR1 * (mt[mti-1] ^ (mt[mti-1] >>> 30)) + mti);
		}
		// ---- End Mersenne Twister Algorithm ----
	}

	/**
	 * This method resets the state of this instance using the 64
	 * bits of seed data provided.  Note that if the same seed data
	 * is passed to two different instances of MTRandom (both of
	 * which share the same compatibility state) then the sequence
	 * of numbers generated by both instances will be identical.
	 * <p>
	 * If this instance was initialised in 'compatibility' mode then
	 * this method will only use the lower 32 bits of any seed value
	 * passed in and will match the behaviour of the original C code
	 * exactly with respect to state initialisation.
	 * 
	 * @param seed The 64 bit value used to initialise the random
	 * number generator state. 
	 */
	@Override
	public final  void setSeed(long seed) {
		if (compat) {
			synchronized(this){
				setSeed((int)seed);	
			}
			
		} else {

			// Annoying runtime check for initialisation of internal data
			// caused by java.util.Random invoking setSeed() during init.
			// This is unavoidable because no fields in our instance will
			// have been initialised at this point, not even if the code
			// were placed at the declaration of the member variable.
			int[] ibuf = new int[2];

			ibuf[0] = (int)seed;
			ibuf[1] = (int)(seed >>> 32);
			setSeed(ibuf);
		}
	}

	/**
	 * This method resets the state of this instance using the byte
	 * array of seed data provided.  Note that calling this method
	 * is equivalent to calling "setSeed(pack(buf))" and in particular
	 * will result in a new integer array being generated during the
	 * call.  If you wish to retain this seed data to allow the pseudo
	 * random sequence to be restarted then it would be more efficient
	 * to use the "pack()" method to convert it into an integer array
	 * first and then use that to re-seed the instance.  The behaviour
	 * of the class will be the same in both cases but it will be more
	 * efficient.
	 *
	 * @param buf The non-empty byte array of seed information.
	 * @throws NullPointerException if the buffer is null.
	 * @throws IllegalArgumentException if the buffer has zero length.
	 */
	public final void setSeed(byte[] buf) {
		setSeed(pack(buf));
	}

	/**
	 * This method resets the state of this instance using the integer
	 * array of seed data provided.  This is the canonical way of
	 * resetting the pseudo random number sequence.
	 * 
	 * @param buf The non-empty integer array of seed information.
	 * @throws NullPointerException if the buffer is null.
	 * @throws IllegalArgumentException if the buffer has zero length.
	 */
	public final synchronized void setSeed(int[] buf) {
		int length = buf.length;
		if (length == 0) throw new IllegalArgumentException("Seed buffer may not be empty");
		// ---- Begin Mersenne Twister Algorithm ----
		int i = 1, j = 0, k = (N > length ? N : length);
		setSeed(MAGIC_SEED);
		for (; k > 0; k--) {
			mt[i] = (mt[i] ^ ((mt[i-1] ^ (mt[i-1] >>> 30)) * MAGIC_FACTOR2)) + buf[j] + j;
			i++; j++;
			if (i >= N) { mt[0] = mt[N-1]; i = 1; }
			if (j >= length) j = 0;
		}
		for (k = N-1; k > 0; k--) {
			mt[i] = (mt[i] ^ ((mt[i-1] ^ (mt[i-1] >>> 30)) * MAGIC_FACTOR3)) - i;
			i++;
			if (i >= N) { mt[0] = mt[N-1]; i = 1; }
		}
		mt[0] = UPPER_MASK; // MSB is 1; assuring non-zero initial array
		// ---- End Mersenne Twister Algorithm ----
	}

	/**
	 * This method forms the basis for generating a pseudo random number
	 * sequence from this class.  If given a value of 32, this method
	 * behaves identically to the genrand_int32 function in the original
	 * C code and ensures that using the standard nextInt() function
	 * (inherited from Random) we are able to replicate behaviour exactly.
	 * <p>
	 * Note that where the number of bits requested is not equal to 32
	 * then bits will simply be masked out from the top of the returned
	 * integer value.  That is to say that:
	 * <pre>
	 * mt.setSeed(12345);
	 * int foo = mt.nextInt(16) + (mt.nextInt(16) << 16);</pre>
	 * will not give the same result as
	 * <pre>
	 * mt.setSeed(12345);
	 * int foo = mt.nextInt(32);</pre>
	 * 
	 * @param bits The number of significant bits desired in the output.
	 * @return The next value in the pseudo random sequence with the
	 * specified number of bits in the lower part of the integer.
	 */
	@Override
	protected final synchronized int next(int bits) {
		// ---- Begin Mersenne Twister Algorithm ----
		int y, kk;
		if (mti >= N) {             // generate N words at one time

			// In the original C implementation, mti is checked here
			// to determine if initialisation has occurred; if not
			// it initialises this instance with DEFAULT_SEED (5489).
			// This is no longer necessary as initialisation of the
			// Java instance must result in initialisation occurring
			// Use the constructor MTRandom(true) to enable backwards
			// compatible behaviour.
			
			for (kk = 0; kk < N-M; kk++) {
				y = (mt[kk] & UPPER_MASK) | (mt[kk+1] & LOWER_MASK);
				mt[kk] = mt[kk+M] ^ (y >>> 1) ^ MAGIC[y & 0x1];
			}
			for (;kk < N-1; kk++) {
				y = (mt[kk] & UPPER_MASK) | (mt[kk+1] & LOWER_MASK);
				mt[kk] = mt[kk+(M-N)] ^ (y >>> 1) ^ MAGIC[y & 0x1];
			}
			y = (mt[N-1] & UPPER_MASK) | (mt[0] & LOWER_MASK);
			mt[N-1] = mt[M-1] ^ (y >>> 1) ^ MAGIC[y & 0x1];

			mti = 0;
		}
  
		y = mt[mti++];

		// Tempering
		y ^= (y >>> 11);
		y ^= (y << 7) & MAGIC_MASK1;
		y ^= (y << 15) & MAGIC_MASK2;
		y ^= (y >>> 18);
		// ---- End Mersenne Twister Algorithm ----
		return (y >>> (32-bits));
	}

	// This is a fairly obscure little code section to pack a
	// byte[] into an int[] in little endian ordering.  

	/**
	 * This simply utility method can be used in cases where a byte
	 * array of seed data is to be used to repeatedly re-seed the
	 * random number sequence.  By packing the byte array into an
	 * integer array first, using this method, and then invoking
	 * setSeed() with that; it removes the need to re-pack the byte
	 * array each time setSeed() is called.
	 * <p>
	 * If the length of the byte array is not a multiple of 4 then
	 * it is implicitly padded with zeros as necessary.  For example:
	 * <pre>    byte[] { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06 }</pre>
	 * becomes
	 * <pre>    int[]  { 0x04030201, 0x00000605 }</pre>
	 * <p>
	 * Note that this method will not complain if the given byte array
	 * is empty and will produce an empty integer array, but the
	 * setSeed() method will throw an exception if the empty integer
	 * array is passed to it.
	 * 
	 * @param buf The non-null byte array to be packed.
	 * @return A non-null integer array of the packed bytes.
	 * @throws NullPointerException if the given byte array is null.
	 */
	public static int[] pack(byte[] buf) {
		int k, blen = buf.length, ilen = ((buf.length+3) >>> 2);
		int[] ibuf = new int[ilen];
		for (int n = 0; n < ilen; n++) {
			int m = (n+1) << 2;
			if (m > blen) m = blen;
			for (k = buf[--m]&0xff; (m & 0x3) != 0; k = (k << 8) | buf[--m]&0xff);
			ibuf[n] = k;
		}
		return ibuf;
	}
}
